1. Field of the Invention
The field of art to which the claimed invention pertains is solid bed adsorptive separation. More specifically, the claimed invention relates to a process for the separation of di-substituted benzene isomers from a feed mixture comprising such isomers, which process employs a solid adsorbent which selectively removes the paraisomer from the feed mixture, thereby producing a fluid raffinate stream comprising a mixture of the nonadsorbed isomers.
2. Description of the Background Information
It is well known in the separation art that certain crystalline aluminosilicates can be used to separate hydrocarbon species from mixtures thereof. The separation of normal paraffins from branched-chained paraffins, for example, can be accomplished by using a type A zeolite which has pore openings from 3 to about 5 angstroms. Such a separation process is disclosed in U.S. Pat. Nos. 2,985,589 and 3,201 ,491. These adsorbents allow a separation based on the physical size differences in the molecules by allowing the smaller or normal hydrocarbons to be passed into the cavities within the zeolitic adsorbent, while excluding the larger or branched chain molecules.
In addition to separating hydrocarbon types, the type X or type Y zeolites have also been employed in processes to separate individual hydrocarbon isomers. In the process described in U.S. Pat. No. 3,114,782, for example, a particular zeolite is used as an adsorbent to separate alkyl-trisubstituted benzene; and in U.S. Pat. No. 3,668,267, a particular zeolite is used to separate specific alkyl-substituted naphthalenes. In processes described in U.S. Pat. Nos. 3,558,732; 3,686,342 and 3,997,620, adsorbents comprising particular zeolites are used to separate para-xylene from feed mixtures comprising para-xylene and at least one other xyklene isomer by selectively adsorbing para-xylene over the other xylene isomers. In the last mentioned processes, the adsorbents used are para-xylene selective; para-xylene is selectively adsorbed and recovered as an extract component while the rest of the xylenes and ethylbenzenes are all relatively unadsorbed with respect to para-xylene and are recovered as raffinate components. Also, in the last mentioned processes, the adsorption and desorption may be continuously in a simulated moving bed countercurrent flow system, the operating principles and sequence of which are described in U.S. Pat. No. 2,985,589.
The effectiveness of type X and Y zeolites for separating isomers of nitrotoluene is disclosed in U.S. Pat. No. 4,270,013. The highest silica/alumina mole ratio obtained with this type of zeolite, i.e. type Y, is 8.
I have discovered that a crystalline silica adsorbent having a silica/alumina mole ratio of at least 12 is highly effective in separating the para isomer of certain di-substituted benzenes from the other isomers, particularly when used with specific desorbent materials.